rt，但是洛谷貌似没有原题，题目大意就是给定一张图求出有哪些线段被擦除之后整张图变为一个二分图

目前是痛苦 RE，求助大佬 qwq

#include<bits/stdc++.h>
using namespace std;

const int nr = 1e6 + 1;
int n, m, cnt, dep[nr], lg[nr], anc[nr][20], dif[nr][2], f[nr], size[nr];
struct Edge { int u, v; } e[nr];
bool inmst[nr], inans[nr];
vector<int> adj[nr];

int find(int x)
{
	if (f[x] == x) return x;
	return f[x] = find(f[x]);
}

void merge(int x, int y)
{
	int fx = find(x), fy = find(y);
	if (size[x] > size[y]) swap(x, y);
	f[fx] = fy, size[fy] += size[fx];
}

void init(int x, int ft)
{
	dep[x] = dep[ft] + 1;
	anc[x][0] = ft;
	for (int i = 1; i <= lg[dep[x]]; i++)
		anc[x][i] = anc[anc[x][i - 1]][i - 1];
	for (int i = 0; i < adj[x].size(); i++)
		if (adj[x][i] != ft) init(adj[x][i], x);
}

int lca(int x, int y)
{
	if (dep[x] < dep[y]) swap(x, y);
	while (dep[x] > dep[y]) x = anc[x][lg[dep[x] - dep[y]]];
	if (x == y) return x;
	for (int i = lg[dep[x]]; i >= 0; i--)
		if (anc[x][i] != anc[y][i]) x = anc[x][i], y = anc[y][i];
	return anc[x][0];
}

int dis(int x, int y, int _lca)
{ return dep[x] + dep[y] - 2 * dep[_lca] + 1; }

void dfs(int x, int ft)
{
	for (int i = 0; i < adj[x].size(); i++)
		if (adj[x][i] != ft)
		{
			dfs(adj[x][i], x);
			dif[x][0] += dif[adj[x][i]][0];
			dif[x][1] += dif[adj[x][i]][1];
		}
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr), cout.tie(nullptr);
	cin >> n >> m;
	for (int i = 1; i <= m; i++)
	{
		cin >> e[i].u >> e[i].v;
		adj[e[i].u].push_back(e[i].v);
		adj[e[i].v].push_back(e[i].u);
	}
	for (int i = 1; i <= n; i++) f[i] = i, size[i] = 1;
	for (int i = 1; i <= m; i++)
		if (find(e[i].u) != find(e[i].v))
		{
			merge(e[i].u, e[i].v);
			inmst[i] = true;
		}
	for (int i = 1; i <= m; i++)
		if (inmst[i]) adj[e[i].u].push_back(e[i].v), adj[e[i].v].push_back(e[i].u);
	lg[0] = -1;
	for (int i = 1; i <= n; i++) lg[i] = lg[i >> 1] + 1;
	init(1, 0);
	for (int i = 1; i <= m; i++)
		if (!inmst[i])
		{
			int _lca = lca(e[i].u, e[i].v);
			int _dis = dis(e[i].u, e[i].v, _lca);
			if (_dis % 2 == 0) dif[e[i].u][0]++, dif[e[i].v][0]++, dif[_lca][0] -= 2;
			else dif[e[i].u][1]++, dif[e[i].v][1]++, dif[_lca][1] -= 2, cnt++;
		}
	dfs(1, 0);
	if (cnt == 0)
	{
		cout << m << '\n';
		for (int i = 1; i <= m; i++) cout << i << " ";
		return 0;
	}
	int ans = 0;
	for (int i = 1; i <= m; i++)
	{
		int pos = (dep[e[i].u] > dep[e[i].v] ? e[i].u : e[i].v);
		if ((inmst[i] || !inmst[i] && cnt == 1) && dif[pos][0] == 0 && dif[pos][1] == cnt) ans++, inans[i] = true;
	}
	cout << ans << '\n';
	for (int i = 1; i <= m; i++) if (inans[i]) cout << i << " ";
	return 0;
}